1. Field of the Invention
The invention relates to a high-performance vibration filter for use as a support for apparatus that has to be protected from the most minute ambient vibrations, such as scientific instruments and high precision manufacturing equipment. This filter is intended to absorb, or attenuate, linear (as opposed to rotary) vibrations down to a remaining amplitude of less than 10.sup.-11 m.
2. Description of Related Art
Vibration filters known to date in the art are essentially of two types. The first type relies on a comparatively large mass which accordingly has a high resistance against vibration as well as a low resonance frequency. Filters of this type are mainly used as supports for high-precision balances and as benches for optical experiments, for example. The second type of vibration filter uses elastic elements to absorb ambient vibrations, namely rubber blocks or springs. Applications for the second type of filter are in the suspension of vibrating engines, as in motorcars, and in high-fidelity turntables.
The filter to be described pertains to the second type of vibration filter in that it makes use of a specific arrangement of elastic elements, springs and others, in groups having different resonant frequencies.
In addition to the attunation of vibrations achieved by these elastic elements, the invention comtemplates the employment of eddy current brakes having the characteristics of considerably lowering the resonant frequency of the system in which they are used.
Except perhaps for the high precision balances mentioned above, vibration damping known in the art has generally to do with vibrations of an amplitude in the 10.sup.-6 m range, and even for record players the requirements do not exceed 10.sup.-8 m. Yet modern surface physics investigates objects of atomic size, and the instruments used in these investigations have to be protected from vibrations on the order of atomic magnitude so as not to jeopardize their inspection and measurement.
A similar, yet reverse problem exists in seismology where ambient vibrations are to be investigated, namely the movements of the earth. To obtain meaningful results, seisometers usually comprise means for reducing the resonant frequency of the seismometer itself. From "Elemente der Seismologie und Seismometrie" by E. F. Sawarenski and D. P. Kirnos, Berlin 1960, p. 361, it is known to employ a copper or aluminum bar fixed to the seismometer pendulum and diving into the gap of a permanent magnet of the horse shoe type. Movement of the pendulum and, hence, of the bar causes the generation of Foucault eddy currents in the bar which tend to oppose the pendulum's movements. These eddy current attenuators are not designed for reducing external vibrations as this would be against the purpose of seismometry.
This is also true for the low-frequency seismometer described in U.S. Pat. No. 3,412,376 Johnson in which a pair of balanced coils of a masscoil assembly are concentrically suspended in annular air gaps of a permanent magnetic field by means of a spring suspension system. The attenuation is here achieved by electromagnetic means not present in the vibration filter of the present invention, and the spring suspension system is entirely different.